The primary goal is to test the hypothesis that oral supplements of calcium and vitamin D both singly and in combination reduce the rate of bone loss and architectural change at the hip in men and women over the age of sixty- five. Secondary goals are to examine the mechanisms through which calcium and vitamin D might alter bone mass at the hip, to seek corroborative evidence of a beneficial effect on the fracture incidence, and to establish clinically useful predictors of bone mass and architecture at the hip. These goals will be achieved by studying 450 subjects drawn from two residential homes for the elderly and from free-living subjects in the same town, over a five year period. The subjects will be stratified by age, basal plasma calcidiol level and basal dietary calcium intake and randomize to receive orally, either placebo, or calcium supplement or vitamin D, in the form of calcidiol. After two and a half years of treatment the effects on bone mass and architecture will be analyzed. The study will continue fo a further two years to test the effect of the combination of calcium and vitamin D on bone mass with the design of this stage of the study determine by the results obtained at two years. Bone mass at multiple sites of the upper femur and total body calcium will be measured by dual energy x-ray absorptiometry; bone architecture will be assessed from standardized radiographs of the pelvis; incidence of fracture and falls, use of drugs, and calcium food frequency will be obtained at regular interview; serum calcium regulation, bone turnover, vitamin D status, the calcium regulating hormones and sex steroids will be assessed from blood and urine biochemistry; calcium absorption will be measured usin an oral radiocalcium test; the ratio of T4 helper/T8 suppressor cells in peripheral blood will be assessed by cell surface-marker analysis; skin-fol thickness of the dorsal surface of the hand will be measured by calipers; distal and proximal muscle strength assessed from measurements of muscle function. Compliance with treatment will be evaluated by pill counts and biochemistry and safety by a combination of regular interviews and biochemistry.